Syndiotactic 1,2-polybutadiene is a crystalline thermoplastic resin that has a stereoregular structure in which the side chain vinyl groups are located alternately on the opposite sides in relation to the polymeric main chain. Syndiotactic 1,2-polybutadiene is a unique material that exhibits the properties of both plastics and rubber, and therefore it has many uses. For example, films, fibers, and various molded articles can be made from syndiotactic 1,2-polybutadiene. It can also be blended into and co-cured with natural or synthetic rubber.
Syndiotactic 1,2-polybutadiene can be made by solution, emulsion or suspension polymerization. The physical properties of syndiotactic 1,2-polybutadiene are largely determined by its melting temperature and molecular weight. Generally, syndiotactic 1,2-polybutadiene has a melting temperature within the range of about 195.degree. C. to about 215.degree. C., but due to processability considerations, it is generally desirable for syndiotactic 1,2-polybutadiene to have a melting temperature of less than about 195.degree. C. Accordingly, there is a need for means to regulate the melting temperature and molecular weight of syndiotactic 1,2-polybutadiene.
Various transition metal catalyst systems based on cobalt, titanium, vanadium, chromium, and molybdenum for the preparation of syndiotactic 1,2-polybutadiene have been reported. The majority of these catalyst systems, however, have no practical utility because they have low catalytic activity or poor stereoselectivity, and in some cases they produce low molecular weight polymers or partially crosslinked polymers unsuitable for commercial use.
The following two cobalt-based catalyst systems are well known for the preparation of syndiotactic 1,2-polybutadiene on a commercial scale: (1) a system containing cobalt bis(acetylacetonate), triethylaluminum, water, and triphenylphosphine (U.S. Pat. Nos. 3,498,963 and 4,182,813), and (2) a system containing cobalt tris(acetylacetonate), triethylaluminum, and carbon disulfide (U.S. Pat. No. 3,778,424). These cobalt-based catalyst systems also have disadvantages.
The first cobalt catalyst system referenced above yields syndiotactic 1,2-polybutadiene having very low crystallinity. Also, this catalyst system develops sufficient catalytic activity only when halogenated hydrocarbon solvents are used as the polymerization medium, and halogenated solvents present toxicity problems.
The second cobalt catalyst system referenced above uses carbon disulfide as one of the catalyst components. Because of its low flash point, obnoxious smell, high volatility, and toxicity, carbon disulfide is difficult and dangerous to use, and requires expensive safety measures to prevent even minimal amounts escaping into the atmosphere. Furthermore, the syndiotactic 1,2-polybutadiene produced with this cobalt catalyst system has a very high melting temperature of about 200-210.degree. C., which makes it difficult to process the polymer. Although the melting temperature of the syndiotactic 1,2-polybutadiene produced with this cobalt catalyst system can be reduced by employing a catalyst modifier as a fourth catalyst component, the presence of this catalyst modifier has adverse effects on the catalyst activity and polymer yields. Accordingly, many restrictions are required for the industrial utilization of these cobalt-based catalyst systems.
Coordination catalyst systems based on iron-containing compounds, such as the combination of iron(III) acetylacetonate and triethylaluminum, have been known for some time, but they have shown very low catalytic activity and poor stereoselectivity for the polymerization of 1,3-butadiene. The product mixture often contains oligomers, low molecular weight liquid polymers, and partially crosslinked polymers. Therefore, these iron-based catalyst systems have no industrial utility.
Because syndiotactic 1,2-polybutadiene is useful and the catalysts known heretofore in the art have many shortcomings, it would be advantageous to develop a new and significantly improved catalyst composition that has high activity and stereoselectivity for polymerizing 1,3-butadiene into syndiotactic 1,2-polybutadiene. It would be additionally advantageous if that catalyst system was versatile enough to control the melting temperature and molecular weight of the polymerization product.